There are several layers of insult going on here, as one would expect from Draco Malfoy. Let's go through them:
...someone's asked that to the ball?
Using that to refer to a specific human is inherently insulting. It's dehumanising, indicating that the person is an object or an animal. The proper pronoun to use here would have been her, but Malfoy is deliberately using that to insult.
"Not the long-molared Mudblood?”
"Not" in this context is used to express surprise, outrage, or denial. Again, this is an insult - Malfoy suggesting that it's incredible to believe that Hermione was invited. It's also phrased as a rhetorical question - a statement phrased as a question for effect or to make a point. In this case, while Malfoy is stating "I don't believe somebody would invite the long-molared Mudblood", he's phrasing it as if asking for confirmation, to suggest that such a thing is so surprising it needs to be questioned.
"Not the long-molared Mudblood?"
Hermione has always been described as having an overbite, a condition characterised by over-large front teeth. Malfoy is using her large teeth to define her in another insult.
His insult is also technically incorrect, since the molars are the back teeth used for chewing, not the front teeth that cause an overbite. However, it's done deliberately in this case. He's using alliteration, a poetic technique in which two or more words use the same beginning sound or letter - in this case long-molared Mudblood"
Not the long-molared Mudblood?
This, of course, is just the typical Malfoy insult that he normally levels at Hermione.
Overall then - Malfoy is implying that Hermione is not really a person but an animal or thing that he defines as a "long-molared Mudblood".